Method for making steel in a liquid melt-fed electric furnace

ABSTRACT

A method for making steel in an electric furnace, wherein a predetermined amount of liquid melt is fed into the electric furnace. The method comprises the steps of (a) continuously feeding a controlled flow of liquid melt into the furnace without interrupting the heating from the electric arc, (b) continuously injecting a refining gas into the furnace before the C and/or Si content of the metal bath reaches a predetermined value, until the end of the feeding process, and (c) pursuing the injection of refining gas after the predetermined amount of melt has been fed into the furnace, until the target value for the C and/or Si content of the metal bath has been reached.

This application is a national stage of PCT/EP97/03005, filed Jun. 10,1997.

The present invention relates to a method of producing steel in anelectric furnace by charging with molten pig iron.

A high proportion of steel scrap is recycled using electric furnacessuch as arc furnaces. These furnaces make it possible to melt and re-usethe steel scrap treated in this way in order to produce new steelproducts.

Some of the residual elements contained in the steel scrap such ascopper, nickel, etc., cannot be separated from the steel and hence occurin the finished products. This means that the more the steel scrap issubjected to recycle operations, the greater the concentration of theseresidual elements. These elements cause problems for the production ofcertain products such as sheet steel, etc.

One way of reducing the concentration of residual elements in the steelobtained from steel scrap and of simultaneously improving the energyefficiency of the electric furnace consists in adding molten pig iron tothe electric furnace. Now, because of the fairly high carbon and siliconcontent of the molten pig iron (typically 4.5% C and 0.6% Si), chargingwith molten pig iron leads to a considerable increase in theconcentration of these elements in the metal bath. The result of this isa longer refining stage in the metal bath in order to reduce the carbonand silicon concentrations in the metal bath to target values, which aregenerally very low, e.g. for the carbon concentration the target valueis between 0.05% and 0.1%.

To achieve this, traditional charging methods, after the charging withpig iron, inject a refining gas, oxygen for example, in order to reducethe carbon and silicon concentrations. With the concentrations of theseelements being quite high, the rate at which oxygen is supplied must bemoderated in order to avoid the disiliconising and decarburisingreactions occurring too violently. In fact, in the presence of highcarbon and silicon concentrations, the injected oxygen reacts veryviolently at the point of impact in the metal bath, leading locally to avery abrupt release of energy and of the reaction gas, such as CO forexample. It is obvious that such a violent reaction is accompanied bysplashes of steel and pig iron which risk fouling and damaging thecooling panels lining the inside of the furnace. Hence the need toreduce the rate at which oxygen is supplied in order to moderate thedynamics of the refining reaction.

However, due to the limited supply rate of oxygen during the refining,the latter operation takes quite a long time and, above a certainquantity of charged molten pig iron, it forms the limiting factor on theduration of a melting cycle in the furnace. In order to improve theperformance of the arc furnace a regards its productivity, i.e. Toreduce the duration of a melting cycle, it is therefore essential toreduce the duration of the refining in the metal bath.

The document EP-A-0 630 977 describes a process for the treatment ofmolten pig iron in a converter equipped with at least one electrode. Itrelates to a process in which the total amount of pig iron is chargedinto the converter before the electric arc is activated.

The object of the present invention is to propose a method of producingsteel in an electric furnace by charging with molten pig iron whichenables the duration of a melting cycle to be reduced.

In conformity with the invention, this objective is achieved by a methodfor producing steel in an electric furnace, in which a quantiityquantity of scrap is charged into the electric furnace and molten by useof an electric arc a predetermined quantity of molten pig iron ischarged into the electric furnace, after a part of the scrap is molten,and a refining gas is injected into the furnace after the plannedquantity of pig iron is charged until a target value of theconcentration of carbon and/or silicon in the metal bath is reached.

The quantity of molten pig iron is charged continuously and at acontrolled rate without interruption of the heating by the electric arc,and injection of the refining gas into the furnace starts during thecontinuous charging before the concentration of carbon and/or of siliconin the metal bath has reached a predetermined limiting value, theinjection taking place continuously until the end of the charging, andthe target value of the concentration of carbon and/or silicon in themetal bath is reached.

This method has the advantage, firstly, that the charging is carried outwithout switching off the power supply, i.e. without interruption of theheating by the electric arc. Consequently, the melting of the steelscrap is not interrupted and is carried out more rapidly than intraditional methods of charging with molten pig iron. Secondly, therefining by injection of a gas begins before the end of the charging,i.e. at a time which is earlier than in traditional charging methods. Asa result of this, the duration of a melting cycle is reduced, eventhough the rate of injection of the gas is not increased.

Since the refining begins before the end of the charging, this methodalso make it possible to reduce the maximum carbon and/or siliconconcentration in the metal bath during a melting cycle by an adjustmentof the rates of charging and of gas injection. At the beginning of therefining, the concentration, for example of carbon, in the metal bath isin fact significantly lower than that obtained in traditional methods inwhich the refining begins only after charging with the total quantity ofmolten pig iron (it is the same for the silicon concentration).Moreover, at least a part of the carbon in the bath is oxidised as it issupplied, so that the increase in the carbon concentration in the metalbath as charging proceeds is substantially reduced and so that itsconcentration does not exceed a predetermined limiting value, which forthe carbon concentration for example is less than 2%, and preferablyless than 1.5%. The silicon concentration behaves in the same way but ona reduced scale. The predetermined limiting value for the siliconconcentration is less than 0.3% for example, preferably less than 0.2%.

The carbon and silicon concentrations being limited in this way, it ipossible to increase the rate of oxygen supply without the refiningreaction taking place too violently. In effect, because of the limitedlocal amount of silicon and carbon, the refining reaction is no longerlocalised at the point of impact of the gas in the bath but the oxygenis carried intermediately on the iron. After stirring the phases thatare present (metal and slag), the iron oxide produced in this way reactssubsequently with the silicon and the carbon that it encounters atplaces other than the point of injection. The release of the reactiongas, such as CO for example, and the splashes therefore occur moreuniformly over the whole surface of the metal bath and consequently muchless violently. Thus, an increase in the rate of oxygen supply and hencein the speed of refining can be achieved without causing splashes ofsteel and pig iron which are too large and which risk fouling anddamaging the coupling panels lining the inside of the furnace. Themelting cycles of the furnace are thus shortened and the productivity ofthe furnace increases.

It should be noted that the charging with pig iron is achieved withoutstoppage of the heating by the electric arc and that the roof of thefurnace remains closed for the whole duration of the charging. Thelatter is carried out preferably through a lateral opening in thefurnace. Since the roof is closed during the whole melting cycle, inputsof air into the furnace chamber are avoided and the nitrogen input isconsiderably reduced. Moreover, the earlier and continuous refiningleads to a continual washing of the metal bath by the reaction gaseslike CO. Through this washing by CO, the nitrogen dissolved in the metalbath is dissolved in the CO bubbles, which take it out of the metalbath. The nitrogen is then removed from the vessel together with thereaction gas by the furnace exhaust system. Such continuous washing thusleads to very low nitrogen concentrations in the steel produced.

As a result of this, the method according to the invention is perfectlyadapted to the production of quality steels, particularly to that ofvery ductile steels, for which very low nitrogen concentrations arerequired.

The rate of supply of the refining gas and the rate of charging with pigiron are preferably adjusted so that the carbon and/or siliconconcentration in the metal bath no longer increases after the refininghas started. It is possible, for example, to adapt the rate of chargingwith pig iron to the maximum rate of oxygen supply so as to oxidise allthe carbon in the bath as it is supplied. In this way, the carbon andsilicon concentrations in the metal bath may be very preciselycontrolled during a melting cycle and it is possible to limit themaximum concentration to very low values, e.g. for the carbon, to aconcentration of 0.5%

According to a preferred execution of the method, the refining gas isinjected into one of the two quadrants of the furnace, which areopposite the feed opening relating to an electrode of the electricfurnace. In this case, the direction of the gas injection is adjusted sothat a first vertical plane containing the direction of charging and asecond vertical plane containing the direction of injection intersecteach other substantially in the region of the furnace electrode.

The reaction gases, such as CO for example, which are releasedcontinuously during the refining, are more abundant in the region wherethe fluxes of gas and pig iron meet each other than in the neighbouringregions. On leaving the metal bath, these gases displace the nitrogen inthe vessel and create above the surface of the metal bath a protectiveatmosphere against the input of nitrogen into the bath.

Because of the very high temperatures in the neighbourhood of theelectric arc, the presence of nitrogen in this region leads to apreferential nitriding of the metal bath. It is therefore greatlypreferable to direct the fluxes of pig iron and refining gas so thatthey meet each other in the region located below the electric arc. Theprotective atmosphere created in the neighbourhood of the arc isconsequently particularly dense and an input of nitrogen into the bathmay be very effectively prevented.

It should be noted that the charging with molten pig iron can be carriedout with an amount lying between 20% and 60% of the total furnace chargeand that the rate of charging with pig iron is preferably less than 4%of the capacity of the furnace per minute. The rate at which oxygen isinjected per tonne of furnace capacity lies with advantage between 0.5and 1 m³ O₂ per minute.

In what follows, a way of carrying out the method is compared with atraditional charging method, using an example illustrated by FIGS. 1 and2. These show:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1: the variations with time of the electric power, of the quantityof molten metal and of the carbon concentration for a traditionalcharging method;

FIG. 2: the variations with time of the electric power, of the quantityof molten metal and of the carbon concentration for a charging methodaccording to the invention.

FIG. 3: the injection of the refining gas in the furnace.

The assumptions common to both methods of charging with molten pig ironare as follows:

capacity of the furnace: 100 t+20 t in the heel;

charging: 66 t steel scrap+44 t molten pig iron, or 40%;

maximum active furnace power; 60 MW;

carbon concentration in the pig iron 4.5%, in the steel scrap 0.5%.

In this example, only the carbon concentration in the metal bath isconsidered. The silicon concentration shows a behaviour essentially thesame as that of the carbon concentration, except that the siliconoxidises before the carbon. As a result, after having reached the targetvalue of the carbon concentration, the silicon is almost eliminated fromthe metal bath.

With a view to facilitating the comparison, a start is first made withthe same maximum rate of oxygen supply for both methods, a rate which isof the order of 4000 m³/h and which corresponds to a decarburising rateof 60 kg carbon/min.

In the traditional method (FIG. 1), the electric furnace is first run atmaximum power to melt a certain quantity of steel scrap. After tenminutes, the electric arc is once again switched off, the furnace coveris removed and the charging with molten pig iron is carried out for 5minutes. After the charging, the furnace cover is replaced and the arcis switched on again. It should be noted that, because of the timerequired to open and close the furnace cover, a 5 minute charge meansthat the furnace is shut down for approximately 10 minutes.

During the charging with pig iron, the mass of molten metal and thecarbon concentration in the metal bath increase linearly because of thecharging rate, and at the end of the charging, the carbon concentrationreaches a value of 3% (the silicon concentration amounts of 0.4%). It ismainly because of these very high silicon and carbon concentrations thatthe rate of oxygen supply must be limited during the refining to one of4000 m³/h. During this refining, which begins after the cover has beenclosed, the carbon concentration is reduced at a substantially linearrate to end up at a value of less than 0.1%.

It should be noted that, because of the quantity of carbon being loadedwith the pig iron and steel scrap and because of the limited oxygensupply rate, the decarburisation lasts for 38 minutes in all. Because itbegins only 20 minutes after the beginning of the melting cycle, thewhole melting cycle lasts for 58 minutes.

In the method according to the invention, illustrated with the help ofFIG. 2, the charging begins after 10 minutes and continues at a rate of3 t/min, i.e. taking about 15 minutes. During the charging, the furnaceremains powered so that the quantity of molten metal in the furnaceincreases not only because of the charging but also because of thesimultaneous melting of the steel scrap. Consequently, the melting ofthe steel scrap ceases 10 minutes sooner than in the method of FIG. 1.

Moreover, the decarburisation, which requires 38 minutes for the samerate of 4000 m³/h, begins a little after the beginning of chargingbefore the carbon concentration in the metal bath exceeds a value of1.5%. Beginning the charging in this way, earlier than in the method ofFIG. 1, already enables the duration of the melting cycle to be reducedby more than 10%. If the maximum oxygen supply rate is now increased,which is possible without the risk of splashes because of the low carbonconcentration in the metal bath, the decarburisation rate increases andthe duration of a melting cycle is reduced still further. As a result,the method according to the invention enables the productivity of anelectric furnace to be improved by at least 10%.

In an optimised version of the continuous charging with pig iron, it ispossible to adapt the rate of charging with pig iron to the maximumsupply rate of the oxygen for decarburisation, so as to oxidise thecarbon as it is supplied to the metal bath. In this way, it is possibleto limit the carbon concentration to values below 0.5%. Because of thislow carbon concentration, the maximum oxygen supply rate can beconsiderably increased so as to increase the decarburisation rate. For apig iron with a carbon concentration of 4.5% the relation between thepig iron supply rate and the oxygen supply rate is then:

q pig iron (t/min)=qO ₂ (m ³ /min)/43.

Such a method with early and optimised charging is represented by abroken line in FIG. 2 for a maximum oxygen supply rate of 5200 m³/h. Thecharging in this case occurs at a rate of 2 t/min. It can be seen thatthe charging begins as soon as the cycle begins and that consequentlythe mass of molten metal increases linearly from the start. The carbonconcentration, on the other hand, remains substantially constant duringthe whole charging and is less than 0.5%. This method makes it possibleto increase productivity by 20% in comparison with traditional chargingmethods.

What is claimed is:
 1. Method of producing steel in an electric furnace,comprising the following steps: charging scrap into an electric furnace;melting the scrap by use of an electric arc; after a part of said scrapis molten, charging molten pig iron into the electric furnacecontinuously at a rate without interruption of heating by the electricarc providing a metal bath; and injecting refining gas into the furnacecontinuously at a rate during the charging of the pig iron, wherein theinjection starts at a point when a quantity of pig iron is charged intothe furnace and before a limit value of carbon and/or siliconconcentration in the metal bath is reached, and ends with the end ofcharging is continued after charging with the planned quantity of pigiron until the target value of the concentration of carbon and/orsilicon in the metal bath is reached.
 2. Method according to claim 1,wherein the rate of charging and the rate of injection of the refininggas are adjusted so that to prevent increase of the carbon and/orsilicon concentration in the metal bath after the beginning of refining.3. Method according to claim 2, wherein the refining gas is injectedinto one of the two quadrants of the furnace, which are both opposite toa feed opening relating to a furnace electrode, in such a way that avertical plane containing the direction of charging and a vertical planecontaining the direction of injection intersect each other substantiallyin the region of the furnace electrode.
 4. Method according to claim 2,wherein the limit value of the carbon concentration in the metal bath isless than 2%.
 5. Method according to claim 2, wherein the quantity ofmolten pig iron is between 20% and 60% of a total charge of the furnace.6. Method according to claim 2, wherein the rate of charging with pigiron is less than 4% of the furnace capacity per minute.
 7. Methodaccording to claim 1, wherein the refining gas is injected into one ofthe two quadrants of the furnace which are both opposite to a feedopening relating to an electrode of the electric furnace, the directionof injection of the gas being such that a first vertical planecontaining the direction of charging and a second vertical planecontaining the direction of injection intersect each other substantiallyin the region of the furnace electrode.
 8. Method according to claim 1,wherein the limit value of the carbon concentration is the metal bath isless than 2%.
 9. Method according to claim 1, wherein the quantity ofmolten pig iron is between 20% and 60% of a total charge of the furnace.10. Method according to claim 1, wherein the rate of charging with pigiron is less than 4% of the furnace capacity per minute.
 11. Methodaccording to claim 1, wherein the rate of injection of the refining gasper tonne of capacity of the furnace is between 0.5 and 1 m³/tonne perminute.
 12. Method according to claim 1, wherein the refining gas is O₂.13. Method of producing steel in an electric furnace, comprising thefollowing steps: providing an electric furnace having an electrode forheating a metal bath with an electric arc and a feed opening relating tothe electrode for injection of refining gas; charging scrap into thefurnace; melting the scrap by use of the electric arc; after a part ofsaid scrap is molten, charging molten pig iron into the electric furnacecontinuously at a controlled rate without interruption of the heating bythe electric arc; and after a predetermined quantity of pig iron ischarged into the furnace, continuously injecting refining gas at anadjustable rate into one of the two quadrants of the furnace which areboth opposite to the feed opening, in such a way that a vertical planecontaining the direction of injection substantially intersects avertical plane containing the direction of charging.
 14. Methodaccording to claim 13, wherein the quantity of molten pig iron isbetween 20% and 60% of a total charge of the furnace.
 15. Methodaccording to claim 14, wherein the injection starts before a limit valueof carbon and/or silicon concentration in the metal bath is reached, andends with the end of charging.
 16. Method according to claim 15, whereinthe rate of charging and the rate of injection of the refining gas arebeing adjusted so that to prevent increase of the carbon and/or siliconconcentration in the metal bath after beginning of refining.
 17. Methodaccording to claim 16, wherein the limit value of the carbonconcentration in the metal bath is less than 2%.
 18. Method according toclaim 17, wherein the rate of charging with pig iron is less than 4% ofthe furnace capacity per minute.
 19. Method according to claim 18,wherein rate of injection of refining gas per tonne of the furnacecapacity is between 0.5 and 1 m³/min.
 20. Method according to claim 19,wherein the limit value of the silicon concentration in the metal bathis less than 0.3%.